We are a grandmother

What's the meaning of the phrase 'We are a grandmother'?

'We have become a grandmother' was UK
Prime Minister Margaret Thatcher's statement to the press in 1989, on the
birth of her first grandchild, Mark Thatcher's son Michael.

What's the origin of the phrase 'We are a grandmother'?

The use of the 'royal
we' (the 'pluralis majestatis' or 'majestic plural') had previously been restricted, as one might expect, to royalty; for example, Queen Victoria's celebrated 'we are not amused'. Its use by a mere prime minister and Thatcher's imperious personal manner were the source of considerable disdain at the time. Thatcher's apparent conceit led to her being described as 'a legend in her own imagination' and to some linguistic
jokes at her expense:

- Why
is Margaret Thatcher like a pound coin?
-
Because she is thick, brassy and thinks she's a sovereign.

Another
quip came in the explanation from an aide
as to why she had the nickname of 'Daggers' Thatcher. An interviewer
asked, 'Is that because she has a reputation for stabbing colleagues
in the back?' 'No, its because she's three stops past Barking.'